The invention relates to an improved process for the preparation of p-hydroxy-benzaldehyde and its derivatives by oxidation of the corresponding p-cresols with oxygen in the presence of chelate complexes of iron and/or manganese.
p-Hydroxy-benzaldehydes are important intermediates for industrial syntheses of flavours, plant-protection products, dyestuffs and pharmaceuticals.
It is possible to prepare p-hydroxy-benzaldehydes by oxidation of p-cresols, the selective oxidation with air oxygen being of industrial interest.
According to EP 0,012,936, p-cresol derivatives can be oxidized in a liquid phase in the presence of a base and a cobalt compound or metallic cobalt and an alcohol as the solvent with oxygen to p-hydroxy-benzaldehydes. For example, using 1 mol % of cobalt chloride and 3 mol of sodium hydroxide, relative to the p-cresol used, a selectivity of p-hydroxybenzaldehyde of 78% is obtained at a conversion of 92%.
According to U.S. Pat. No. 4,453,016, this process is improved by using a salt of the metals cobalt, manganese, nickel or chromium in combination with activated carbon. In the example listed there, 6.7 mol % of cobalt chloride and 6.7% by weight of activated carbon, relative to the p-cresol used, are employed. The addition of activated carbon increases the reaction rate. According to U.S. Pat. No. 4,471,140, the addition of an amine is also active in the same manner.
An improvement in the selectivity compared to the oxidation of p-cresol to p-hydroxy-benzaldehyde with oxygen, which is catalyzed by cobalt, is obtained, according to JP 61/24,535 (1986) by carrying out the oxidation in the presence of a combination from a cobalt and a copper compound. By using a total of 1.5% by weight of cobalt and copper salt, a selectivity of 87% of p-hydroxy-benzaldehyde at a p-cresol conversion of 97% is obtained. According to JP 62/153,240 (1987), the amount of the cobalt compound used can be reduced to one-tenth if the iron ion content is simultaneously maintained under 6 ppm.
According to JP 62/132,836 (1987), the oxidation of p-cresol to p-hydroxy-benzaldehyde is carried out in the presence of a mixture of an iron(III) and a nickel(II) compound, which achieves the establishment of equilibrium more rapidly. The selectivities are 52-81% and the conversions 42-54%. According to JP 62/135,443 (1987), the oxidation is carried out in the presence of iron(III) compounds, preferably iron(III) salts, thus achieving a selectivity of 40-60% at 40-50% of conversion.
The previous processes have the disadvantage that (a) in some cases significant amounts, in most cases about 1 mol %, relative to p-cresol, of the catalytically active heavy metal are used, water-soluble salts being preferably used, and (b) the p-hydroxy-benzaldehyde selectivities are in part only moderate.
In all these processes, during the subsequent aqueous work-up, the heavy metal salts end up in the waste water, which makes this waste water difficult to dispose of and loses the catalyst metals.